Currently, CD-R/RW, DVD-R/RW, DVD+R/RW, and the like are available as optical disks capable of high-density recording. When recording information on these recordable optical disks, a reference clock signal for writes cannot be generated from a reproduction signal used on conventional read-only disks.
Thus, a recordable optical disk is configured as shown in FIG. 8. FIG. 8 shows part of a DVD-R/RW disk surface. Grooves 12 for use to record information are formed spirally on the optical disk (hereinafter interchangeably referred to as a DVD-R/RW) 10, wobbling periodically with a fixed amplitude. They are called wobbles.
The grooves 12 are designed such that large volumes of pit data 14 which represent various information are written into it. Also, notches called land pre-pits (hereinafter referred to as LPP) 18 are provided in each land 16 between a groove 12 and groove 12 as also shown in FIG. 9.
Light reflected by the wobbling grooves 12 irradiated with a laser spot is detected by a four-part photodetector (split-type optical sensor) 20 in a push-pull fashion. Consequently, difference in light quantities is detected. The detected signal is called a wobble signal. A reference clock signal for writes can be generated from the wobble signal.
No address information is superimposed on the wobble signal of the optical disk 10 and an LPP signal obtained from the LPP 18 is used as address information. Where the LPP 18 is formed, the land is discontinuous. Consequently, the quantity of light entering the photodetector 20 increases sharply when the laser spot passes through the discontinuous part and as a result of push-pull operation, a pulsed signal is superimposed as the LPP signal on the wobble signal.
The wobble signal and LPP signal are used to control the rotational speed of the optical disk 10 and provide positional information. Recording/playback apparatus for optical disks 10 must detect the signals with high accuracy.
Generally, signals A, B, C, and D read from the optical disk 10 by the photodetector 20 have the following relationships. The wobble signal and LPP signal are extracted using arithmetic operations based on these relationships. On the optical disk 10 after recording, the A, B, C, and D signals contain an RF signal (waveform signal) component which represents difference in reflectance between marks 12a (i.e., pit data 14 in FIG. 8) and spaces 12b. 
A=RF signal+first wobble signal+LPP signal
B=RF signal+second wobble signal (opposite in phase to the first wobble signal)
C=RF signal+second wobble signal (opposite in phase to the first wobble signal)
D=RF signal+first wobble signal+LPP signal
The RF signals in all the A, B, C, and D signals have the same phase and basically the same amplitude. The wobble signals in the A and D signals are opposite in phase to the wobble signals in the B and C signals. The LPP signal occurs only in the A and D signals (or B and C signals).
Due to these relationships, the arithmetic operation of (A+D)−(B+C) in principle cancels out the equiphase components, i.e., RF signals, leaving only the wobble signals and LPP signals. This is expressed as follows.(A+D)−(B+C)
=(RF signal+first wobble signal+LPP signal+RF signal +first wobble signal+LPP signal)−(RF signal+second wobble signal+RF signal+second wobble signal)
=2×(RF signal+LPP signal+first wobble signal)−2×(RF signal+second wobble signal)
=2×LPP signal+4×first wobble signal (this is because the first wobble signal and second wobble signal are opposite in phase, and thus “first wobble signal=−second wobble signal”)
Actually, however, due to variations in the sensitivity of the photodetector 20 as well as variations in amplifier gain, the A+D signal and B+C signal are not exactly equal in amplitude components of the RF signals. Therefore, the arithmetic operation is performed after equalizing the amplitude components of the RF signals by adjusting the gains of A+D signal and B+C signal. AGC (Automatic Gain Control) amplifiers are often used for the gain adjustment and are configured as in the case of a wobble processing circuit 30 shown in FIG. 11.
In the wobble processing circuit 30, an adder 31 adds the A signal and D signal while an adder 32 adds the B signal and C signal. An AGC amplifier 34 adjusts the gain of the A+D signal outputted from the adder 31 while an AGC amplifier 35 adjusts the gain of the B+C signal outputted from the adder 32. That is, the gains are adjusted in such a way as to equalize the amplitude components of the RF signals between the A+D signal and B+C signal. Then, a subtractor 37 subtracts the B+C signal from the A+D signal to output the (A+D)−(B+C), i.e., the wobble signal and LPP signal.
However, the optical disk 10 is recorded by changing light quantity between the marks 12a and spaces 12b according to recording data. Consequently, the wobble signal and LPP signal cannot be reproduced by simply detecting beams returning from the optical disk 10.
To deal with this situation, an S/H wobble processing circuit 40 is configured by connecting S/H (sample/hold) circuits 41 to 44 to the input side of the wobble processing circuit 30 as shown in FIG. 12.
Operation of the S/H wobble processing circuit 40 will be described with reference to a timing chart shown in FIG. 13. It is assumed here that the optical disk 10 is a compact disk.
First, during recording, in a segment in which recording data shown in (1) is High, i.e., in a segment of High state which specifies a mark 12a to be formed (in a mark segment), a laser beam directed at the optical disk 10 becomes strong corresponding to the High state to provide write power. That is, a waveform of a writing beam shown in (2) goes High, forming a mark 12a on the optical disk 10. In so doing, a waveform of a return beam used to read the A, B, C, and D signals goes High by rising sharply as shown in (3).
On the other hand, in a segment in which the recording data (1) is Low, i.e., in a segment of Low state which specifies a space 12b to be formed (in a space segment), a laser beam directed at the optical disk 10 becomes weak corresponding to the Low state to provide read power. That is, the waveform of the writing beam (2) goes Low. In this case, the waveform of the return beam is Low as shown in (3).
As the photodetector 20 detects the return beam with such a waveform as the one shown in (3), the S/H wobble processing circuit 40 generates the A, B, C, and D signals and inputs them in the S/H circuits 41 to 44. Then, the S/H wobble processing circuit 40 switches the level of an S/H switching signal 46 as shown in (4). Specifically, the S/H wobble processing circuit 40 samples the S/H switching signal 46 in High state in a segment in which the return beam is Low, and holds the S/H switching signal 46 Low in a segment in which the return beam is High. Consequently, output level of each of the S/H circuits 41 to 44 becomes Low as shown in (5). As the outputs are inputted in the adders 31 and 32, the subtractor 37 produces the (A+D)−(B+C) signal as the wobble signal.
In this way, conventional compact optical disks obtain a wobble signal by sampling space segments. The S/H wobble processing circuit 40 is capable of such processing because the wobble signal has a frequency of 22.05 KHz which is much lower than the recording data's frequency of 200 KHz to 720 KHz, and thus reproduction of the wobble signal is not much affected by the S/H process.
Conventional optical disk devices of this type include, for example, the one disclosed in Patent Document 1 described later. The optical disk device disclosed in Patent Document 1 performs sampling to reproduce a CD-R/RW wobble signal only when spaces are encountered, and then obtains a wobble signal by predetermined signal processing.
Besides, optical disk devices which use an LPP signal include, for example, the one disclosed in Patent Document 2 described later. The optical disk device disclosed in Patent Document 2 is mainly intended for DVD-R/RW and obtains the LPP signal by performing signal processing separately for marks and spaces. That is, the marks and spaces are separately subjected to signal processing, levels of the marks in binary terms are extracted by an LPF (Low Pass Filter), and AC voltages are added. Also, levels of the spaces in binary terms are sampled alone and extracted by an LPF, and AC voltages are added. Signals obtained by the additions of the AC voltages are binarized by a comparator, resulting values are ANDed, and thereby an LPP signal for each segment is detected. Finally, the LPP signals are ORed, and thereby LPP signals in both mark and space segments are reproduced.
However, conventional optical disk devices have the following problems. Since the technique disclosed in JP2002-216355A (pp. 5-7, FIG. 1; hereinafter referred to as Patent Document 1) samples only space segments being recorded, if LPP signals which serve as a reference for recording and reproducing operations are inserted as in the case of DVD-R/RW or the like, although the LPP signal recorded in the space segments can be reproduced, the LPP signal recorded in the mark segments cannot be reproduced.
Also, in the case of DVD+R/RW, since there is no land pre-pit and the wobble signal contains address information, the wobble signal must be reproduced with high quality. However, when the wobble signal and recording data are close in frequency as is the case with DVD+R/RW, if space segments alone are sampled, it is difficult to detect the wobble signal and thus it is not possible to reproduce high-quality wobble signal.
The technique disclosed in JP10-283638A (pp. 5-7, FIG. 1; hereinafter referred to as Patent Document 2) can reproduce only the LPP signal, and cannot reproduce the wobble signal.
That is, other means must be used to reproduce the wobble signal. Thus, in view of the above problems, it can be seen that the wobble signal and LPP signal should be reproduced both in mark segments and space segments. During recording on an optical disk, laser intensity varies between mark segments and space segments and naturally return beam intensity varies as well. Thus, in principle, the gain can be switched quickly in sync with return beams. To switch the gain, there is known, for example, a gain-switching wobble processing circuit 50 shown in FIG. 14.
Switching operation of the gain-switching wobble processing circuit 50 will be described with reference to a timing chart in FIG. 15. It is assumed here that the optical disk is a compact disk.
During recording, a waveform of a writing beam shown in (2) becomes High or Low depending on whether recording data shown in (1) is High or Low. Consequently, a waveform of a return beam goes High by rising sharply as shown in (3).
As a photodetector detects a return beam with such a waveform, the gain-switching wobble processing circuit 50 generates A, B, C, and D signals and inputs them in gain switching circuits 51 to 54. Then, the gain-switching wobble processing circuit 50 switches the level of a gain-switching signal 56 as shown in (4). Specifically, the gain-switching wobble processing circuit 50 switches the gain by setting the switching signal 56 High (increase the gain) in a segment in which the waveform of the return beam is Low and setting the switching signal 56 Low (decreases the gain) in a segment in which the waveform of the return beam is High. Consequently, the output level of the gain switching circuits 51 to 54 becomes constant in all segments as shown in (5). As the outputs are inputted in adders 31 and 32, a subtractor 37 produces an (A +D)−(B+C) signal as the wobble signal and LPP signal.
However, when the gain is switched quickly, although there is no problem if there is no timing offset between the gain switching and the return beam as in the case of segment SE1, if there is a timing offset between the gain switching and the return beam as in the case of segment SE2, unnecessary pulsed signal components such as those indicated by P1 and P2 are included when the gain is switched, degrading the quality of the wobble signal and LPP signal.
To solve this problem, the gain should be switched very quickly with highly accurate timing. Actually, however, it is difficult to achieve such timing adjustment in view of temperature variations and the like.
A write-once medium such as -R or +R contains much noise at the beginning of recording segments due to a large input signal or electrical overshoot. This may degrade signal quality.
The present invention has been made in view of the above problems and has an object to provide an optical disk device, circuit for an optical disk device, wobble signal reproduction method, and land pre-pit signal reproduction method which can reproduce a wobble signal and LPP signal with high quality.